Technical Abstract:
It has been proposed that low fetal hematocrit (HC) contributes to the poor survival of crowded fetuses and that placental efficiency (PE, fetal weight (FW)/placental weight (PW)) can be used to select for uterine capacity. These traits were measured in control (C), ovulation rate (OR), and uterine capacity (UC) selected lines. HC was greater (P<.01) in UC (38.5 +/- .25%) and less (P<.01) in OR (35.4 +/- .22%) than C (37.2 +/- .23%). PE was greater (P<.05) in OR (4.66 +/- .05) than UC and C (4.46 +/- .06 and 4.36 +/- .05), which did not differ. Heritabilites (h2) for HC, FW, PW, and PE were .11, .05, .18 and .29, respectively. Thus, PE did not increase in UC. In contrast selection for higher HC may increase uterine capacity. To explore this possibility, HC was measured on 5 piglets from each of 3 litters at birth, 7 and 24 h after birth. HC was 44.5 +/- .9, 34.3 +/- .9 and 31.7 +/- .9% at birth, 7 h (P<.05), and 24 h. HC was then determined at 12 to 24 h after birth on piglets from the three lines (106 litters). HC was lower (P<.01) for UC (36.2 +/- .6%) and OR (36.4 +/- .5%) than C (38.6 +/- .5) and h2 for HC was .03 +/- .14. Thus, (1) selection for UC did not change PE; (2) selection for UC increased fetal HC and decreased neonatal HC; (3) HC falls during the early postnatal period; (4) h2 of fetal HC is positive but low, and (5) neonatal HC is unlikely to be useful as a selection tool. Alternative methods (e.g., genetic markers, transgenics) to alter fetal HC are needed and may improve fetal survival and litter size.